The present invention relates in general to analog to digital converters, and in particular, to a new and useful analog to digital converter which utilizes the output of a frequency to digital converter to accurately and inexpensively convert analog signals into digital signals.
Microprocessor based digital systems are often used as part of process control and data collection equipment. In these systems, it is necessary to convert analog signals from the process to digital information which can be manipulated by the microprocessor. One of the more common methods is to use a multiplexer which is controlled by the microprocessor to pick an analog signal to be sent to a signal conditioner. This signal conditioner increases or decreases the signal level to that level needed by an analog to digital converter chip. This chip is used to convert the analog signal at its input into a digital number whose value is directly proportional to the voltage at its input. This chip tends to be expensive and the degree of resolution, that is the largest increment of the analog signal that can be converted into a single digital number, is dependent on the chip. In other words, if a lower analog signal level must be resolved, then the signal conditioner will have to increase the signal level to the input of the analog to digital converter chip. In this case, the firmware, that is, programs that tell the microprocessor how to read this digital number and which signal input the microprocessor is looking at, could not increase the resolution.
Two other types of analog to digital converters use a multiplexer with signal conditioning and a voltage to frequency converter. The microprocessor tells the multiplexer which analog signal to pick up and a direct frequency is read by the microprocessor during that time period. The first of these converters counts the number of pulses during a set time as shown in FIG. 1. The pulse count is divided by the time period to give the frequency. A problem associated with this method is that a time segment before the first pulse (tl) and after the last pulse (t2) cause an error in the time period. The lower the frequency the worse this error gets. If a large number of these periods are averaged together this error will decrease, but the microprocessor spends more time looking at its inputs. The maximum frequency can be raised to compensate for this error. The more pulses in the time period, the smaller times (tl) and (t2) become which cause less error in the period. This, however, has the same effect as where the processor spends too much time looking at the inputs and not doing anything else.
The second method, shown in FIG. 2, is also sometimes used. In this method, the first pulse time is noted, the last pulse time is noted and the number of pulses in between are counted.
The period is defined by the last pulse time minus the first pulse time. The total count of pulses is divided by the time period resulting in the frequency of the signal input This method has two problems. It is desirable to use a period of 16.67 milliseconds because at 16.67 milliseconds the noise associated with AC power lines is filtered out. The period used by this method, however, varies and is not set. Because of the variable period, the firmware routines are harder to design and the processor is again spending too much time looking at its inputs.
U.S. Pat. No. 4,016,557 to Zitelli et al discloses an analog system including a multiplexer which selectively extends analog signals representing plural process variables to an analog to digital (A/D) converter. The A/D converter converts each input analog signal into a digital representation in the form of a pulse count corresponding to the amplitude of the analog signal. This reference uses a variable gain approach where the system must be calibrated for each gain. There is no disclosure containing details of the A/D converter.
U.S. Pat. No. 4,086,656 to Brown discloses an analog to digital integrator which uses a voltage to pulse frequency converter. This reference does not treat problems associated with a loss of resolution nor does it utilize a multiplexer. While the use of different reference voltages is disclosed, there is no teaching on how the effects on accuracy of an external multiplexer would be compensated for.
U.S. Pat. No. 4,118,696 to Warther discloses an A/D converter employing a precision reference voltage. The circuits of the converter include a voltage/frequency oscillator which generates a pulse train corresponding to the input of an analog signal applied thereto. Although the number of pulses within a fixed time period is counted, the time period between first and last pulses is not taken nor is a multiplexer or microprocessor utilized. The errors illustrated in FIG. 1 of the present application would thus occur in this reference.
U.S. Pat. No. 4,309,767 to Andow et al discloses a monitoring system including a multiplexer and A/D converter which apply signals to a digital processing system. There is no teaching concerning how the A/D conversion is conducted within the converter.
U.S. Pat. No. 4,527,148 to Kuboki et al discloses an analog to digital converter in which analog signals are multiplexed, and converted to digital form for processing by a microprocessor. The analog circuits do not, however, employ a voltage to frequency conversion operation.
U.S. Pat. No. 4,547,724 to Beazley et al discloses multiplexing using a three position switch for applying three different voltages to a voltage to frequency converter. No mention is made concerning the resolution problem or any solution to that problem.